It is well-known that the size-divergence product (SDP) of a light beam is an optical invariance throughout an optical system. It is also well-known that the properties of light beam emitted from edge emitting semiconductor laser, or diode laser, are quite different from other laser beams, in that its SDP in lateral direction, which is in parallel to the direction of its PN junction or quantum well plane, and SDP in transverse directions, which is perpendicular to the direction of PN junction or quantum well plane, are quite different. This huge difference in SDPs along the two directions causes substantial difficulties in diode laser applications where a uniform beam property is required, for example focusing beam into a small round spot.
To overcome the difficulty caused by the inherent asymmetric property of edge emitting diode laser, there are a number of beam shaping techniques to create round beam spot from line-like emission out of edge emitting diode laser. Techniques have been developed which reformed the laser beam, such as a china patent (ZL 02 2 53490.3, issued on Aug. 13, 2003).
This invention provided an apparatus for converting light beams includes a light source that can emit a light beam array and multiple roof reflectors. Each roof has two reflecting surfaces intersecting along a common vertex edge to form a dihedral angle between them. The dihedral angle is the integer times of 45°, but not greater than 90°. The roof reflectors are fixed on a substrate having a cardan under its lower surface. The horizontal and vertical components of the light beam orientation emitted by light source are interchanged. The performance of the conversion apparatus doesn't relate to the wavelength of the light beam.
By using the apparatus for converting light beams, each of the light beams is retro-reflected and rotated by 90° around its propagating direction. So the SDP in lateral and longitudinal directions can be adjusted that it is possible to focus the light beams into a small spot. But the propagating direction of the reflected light beam must be different from the incident light beam. To solve this problem, the roof reflectors can be rotated by 45° around direction X, so that the light beam reflected by the roof reflectors can be rotated by 90° around direction X. It means that the reflected light beam propagate in direction Y and rotate by 45° around direction Z. For all these reasons, there is a difference of light path between adjacent reflected light beams.